Which device is specific to a steam boiler?

The steam pressure gauge is specific to a steam boiler because it is designed to monitor and display the pressure of steam within the boiler system. This device is essential in ensuring that the steam pressure remains within the safe and operational limits, which is crucial for both safety and efficiency in steam systems.

In a steam boiler, proper pressure management is vital, as it influences the boiler's efficiency, performance, and the safety of the entire system. The steam pressure gauge allows operators to make informed decisions regarding the operation and maintenance of the boiler, thereby preventing over-pressurization, which can lead to equipment failure or unsafe conditions.

Other devices mentioned serve different functions. For example, an expansion tank is primarily used in hot water systems to accommodate changes in water volume due to temperature fluctuations, while a hot water control valve regulates the flow of hot water rather than steam. A flow switch is used to detect the presence or absence of flow in a system, which is not segment-specific to steam boilers as it is also applicable in other types of fluid systems.